Methods of measuring electric power are well known where time based current (It) and voltage (Vt) delivered to a load, for example an electric motor, are measured as a function of time. Then the real power Pr may be computed as:Pr=Vt*It cos θwhere θ is the phase angle between Vt and It 
Apparent power Pa may be computed as:Pa=Vr*Ir where Vr and Ir are the root mean square (rms) values of voltage andcurrent respectively.
Additionally many commercially available devices use direct or microprocessor methods to provide these computations.
In general, equipment used to monitor power, voltage or current supplied to a motor is used to interrupt the supply in the event that a fault condition develops. A typical example of this is a thermal overload device where the time duration of current is used to estimate the load demand conditions. Generally this estimate may be based on the value of Irms2t as a representation of energy being above a specific value to prevent continued operation.
Electrical sensors of various types are used to detect the current flowing through a conductor. Such sensors include, for example, the well known Hall Effect Device (HED) sensor that produces an output voltage indicative of the current magnitude as well as more conventional current transformer and series resistor.
In equipment such as the commercially available load monitor from Load Controls Inc. (www.loadcontrols.com), a Hall Effect Device may be used to measure delivered real power and allow a control supervisory system to manage the motor operation and protection.
Hall Effect devices for power measurement are commercially available for example the Melexis part number MLX90242 or the Allegro part number ACS712. It is also well known that by supplying the Hall Effect device with a voltage representative of Vt, then the signal output from the Hall Effect Device is representative of the real power.
Hall Effect Devices may be used to sense magnetic flux resulting from a flow of current through a conductor. Some of these known devices have used a flux concentrator to concentrate magnetic flux emanating from the flow of current through a conductor. U.S. Pat. Nos. 4,587,509; and 4,616,207 disclose such a current sensing apparatus.
It is also known to measure the current in a conductor with one or two appropriately placed Hall sensors that measure flux density near the conductor and to convert the same to a signal proportional to current as disclosed in U.S. Pat. No. 6,130,599, U.S. Pat. No. 6,271,656, U.S. Pat. No. 6,642,704 and U.S. Pat. No. 6,731,105.
Another example is Eaton's commercially available ECS7 microprocessor based current monitor. This product may be used to measure delivered current and allow a control supervisory system to manage the motor operation and protection.
U.S. Pat. No. 6,642,704 discloses a current sensor assembly that maintains a pair of magnetic field detectors parallel to one another and closely disposed to an electronic circuit card. Furthermore, the magnetic field detectors are selectively adjustable in order to be attached to a variety of electrical power conductors.
Measurement of current is used in many other situations for a variety of purposes including detection of potential or actual faulty operating conditions of a drum hoist such as overload, or for detection of a fault internal to a hoisting motor such as U.S. Pat. No. 6,966,544 titled “Hoist Apparatus” which applies to a drum hoist. A further example is found in U.S. Pat. No. 5,896,257 titled “Two Sensors for Over-Current Protection and Current Sensing Configuration Motor Control” which requires multiple current sensors to completely characterize motor operating conditions.